The higher level of PerkPass books into Y class (fare code YPASS), so I'm afraid your booking isn't an error but is the norm. PerkSaver books into W class and the lower level of PerkPass books into L class, if I'm not mistaken. So, I'm afraid your chances of a buy-up are nil although you may be offered to upgrade to WBC for a fee ($250-350 seems the norm) at check-in. Good luck.